The 2.0.0 release removed support for variable argument callbacks. When you acquire
a resource from the pool, your callback must accept two arguments: (err, obj)

Previously this library attempted to determine the arity of the callback, but this resulted
in a variety of issues. This change eliminates these issues, and makes the acquire callback
parameter order consistent with the factory.create callback.

If you are shutting down a long-lived process, you may notice
that node fails to exit for 30 seconds or so. This is a side
effect of the idleTimeoutMillis behavior -- the pool has a
setTimeout() call registered that is in the event loop queue, so
node won't terminate until all resources have timed out, and the pool
stops trying to manage them.

This behavior will be more problematic when you set factory.min > 0,
as the pool will never become empty, and the setTimeout calls will
never end.

In these cases, use the pool.drain() function. This sets the pool
into a "draining" state which will gracefully wait until all
idle resources have timed out. For example, you can call:

Pool() accepts an object with these slots:
name : name of pool (string, optional)
create : function that returns a new resource
should call callback() with the created resource
destroy : function that accepts a resource and destroys it
max : maximum number of resources to create at any given time
optional (default=1)
min : minimum number of resources to keep in pool at any given time
if this is set > max, the pool will silently set the min
to factory.max - 1
optional (default=0)
refreshIdle : boolean that specifies whether idle resources at or below the min threshold
should be destroyed/re-created. optional (default=true)
idleTimeoutMillis : max milliseconds a resource can go unused before it should be destroyed
(default 30000)
reapIntervalMillis : frequency to check for idle resources (default 1000),
returnToHead : boolean, if true the most recently released resources will be the first to be allocated.
This in effect turns the pool's behaviour from a queue into a stack. optional (default false)
priorityRange : int between 1 and x - if set, borrowers can specify their
relative priority in the queue if no resources are available.
see example. (default 1)
validate : function that accepts a pooled resource and returns true if the resource
is OK to use, or false if the object is invalid. Invalid objects will be destroyed.
This function is called in acquire() before returning a resource from the pool.
Optional. Default function always returns true.
log : true/false or function -
If a log is a function, it will be called with two parameters:
- log string
- log level ('verbose', 'info', 'warn', 'error')
Else if log is true, verbose log info will be sent to console.log()
Else internal log messages be ignored (this is the default)

The pool now supports optional priority queueing. This becomes relevant when no resources
are available and the caller has to wait. acquire() accepts an optional priority int which
specifies the caller's relative position in the queue.

// create pool with priorityRange of 3

// borrowers can specify a priority 0 to 2

var pool = poolModule.Pool({

name :'mysql',

create : function(callback){

// do something

},

destroy : function(client){

// cleanup. omitted for this example

},

max :10,

idleTimeoutMillis :30000,

priorityRange :3

});

// acquire connection - no priority - will go at front of line (same as high priority)

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